With kind permisson from the author Bob Grimstead and www.pilotweb.aero.Pilot is Britain’s best selling GA magazine, for the latest articles in aviation, aircraft classifieds and pilot training from Pilotweb and Pilot Magazine UK.
Skating on thin air
The time had come for us BOAC cadet pilots to investigate the darker corners of the intimidating Boeing 707’s flight envelope.
This, our first swept-wing jet, was an unforgiving one, so after the slightly scary low-speed handling sortie, we all went up again to experience its high speed characteristics. These were even more daunting. Remember the sound barrier had only been broken a little more than twenty years earlier.
The 707 was the swiftest early jetliner, because of its 35° of sweepback compared with the VC10’s 32.5°, the DC-8’s 30° and the Comet’s minimal 20°. Unfortunately, as well as having the highest cruising Mach number, it necessarily suffered most from the transonic disadvantages. First, we climbed to 35,000ft to sample the reduced aerodynamic damping in that thin air, when flying by hand rather than on autopilot. Many airline pilots have never tried actually flying their aircraft at high altitude. Believe me, no simulator session can replicate the real-world sensitivity to control inputs up there. You are flying very fast (the only reason that
passengers buy tickets) and the air is extremely thin, so you control your aeroplane in pitch with pressures rather than movements of the control column. But, because of your high speed, those pressures are significant, so you need extreme finesse−with the elevator in particular. Forgive the mixed units, but a twentypound push or pull force might be required to move the stick just a couple of millimetres, and that can quickly result in a height excursion of a hundred feet or more. Over the decades, many airline pilots have lost control of their aeroplanes at high altitude after their autopilot disconnected, primarily because of their unfamiliarity with the challenges of manual flying up there. The generic term for these occurrences is ‘jet upset’. Compounding those difficulties is the high-speed characteristic of ‘Mach Tuck’, a phenomenon first reported by Spitfire and Mustang pilots, and the cause of many an airman’s death. As your Mach number increases above roughly 0.85, the wing’s upper shock wave causes movement in the centre of lift both rearwards and outwards, making the aeroplane pitch nose-down. At the same time, shock waves over the tail reduce your elevator’s effectiveness to counter that pitch-down. Thus you experience divergent pitch instability. Most airliners have Mach trimmers to counteract this, but the 707’s was simple and unduplicated, so we had to experience the phenomenon. It was scary, and the high Mach number led to significant buffet while we were overcoming that pitch-down with a careful rearward pull exceeding sixty pounds of force! It was fascinating and educational to go back into the cabin during the other guys’ efforts. If you sat in exactly the right place, about one-third the way back from the leading edge, and looked out along the wing towards its tip, you could just discern that vertical, scimitar-shaped, shock wave discontinuity where the airflow became supersonic. It jiggled and danced like a barely visible sprite back and forth over the chord as our speed varied. Somewhere I still have a monochrome photo of that captivating phenomenon.
Another swept-wing characteristic was Dutch roll−named after that rolling-andyawing gait exhibited by Dutch speed skaters. It is caused by the swept wing’s strong yawroll couple. A problem with all early jetliners, it got worse with increasing height and weight or reducing airspeed. Boeing indeed lost a 707 to fire when trainers were demonstrating Dutch rolling and engines number one, two and four were torn off. Although caused by insufficient yaw stability, we were taught to correct it with a sharp opposing aileron input or two, just as the wings rolled through level. Although only slowly divergent, the 707’s Dutch rolling always needed a counteracting yaw damper, and even with that, it still gently weaved its way around the world. Later versions had a series unit which applied imperceptible corrections, but our early marks had a parallel damper, so the rudder pedals moved with its inputs. Because it would oppose any rudder applied to counteract the swing from a failed engine, it was only engaged after the flaps had been retracted on climb-out, and was disengaged in the event of engine failure or during the landing check on final approach. Many over-loaded neophyte co-pilots would respond to the landing checklist prompt of ‘Yaw Damper’ not with “Disconnected” but “Roger, my damper”.
Finally, to get back down and simultaneously round off our high speed experience, the training captain demonstrated an emergency descent. This was accomplished from cruising flight by closing the thrust levers, raising the speedbrakes and extending the landing gear. The normal speed limit for this was 270kt, but for an emergency decent that increased to 320kt. In cruise our IAS was almost always below this, so it was a quick configuration change and then a positive push over into a precipitous plunge earthwards at Mach .83 and 320kt. This was impressive in the cockpit, where the horizon disappeared above our windscreens; but imagine what it would have been like for passengers. First the ‘rubber jungle’ would drop (all those yellow oxygen masks) as their ears popped. Then the roaring engines would suddenly go silent, there would be a lurch as the speedbrake panels all popped up, with a thunderous airflow rumbling as the gear doors opened and the wheels went down. That would be followed by a massive pitch forwards and a plummeting descent. Good luck staying calm with that lot happening. Nowadays it would be deemed far too dangerous to fly these sorties in a real aeroplane but we had to, because 1970s simulators didn’t have the required fidelity. Plus we just accepted that airline flying was
dangerous. Accidents were commonplace. No fewer than seventy-two airliners crashed with 2,373 deaths in my first full year of
airline flying, back when only one fourteenth of last year’s passengers travelled. Boeing 707s crashed fatally eight and a half times more often than 747s. In 1970 you were six hundred times more likely to die in an airliner crash than today. It was our lives’ work to reduce those accident numbers.